The present invention relates to a method for the oligomerization of ethylene.
Methods for the oligomerization of ethylene using various catalyst compositions are well known in the art. Typically, if very unspecific catalysts are used, a broad product distribution is obtained from C4 to higher olefins and even polymeric materials. Higher linear alpha-olefins and polymeric materials may cause plugging and fouling of the oligomerization reactor and pipings connected therewith. Recently, catalyst compositions for the oligomerization of ethylene have been developed which are more specific to, for example, trimerization or tetramerization, thus resulting in a narrower product distribution, but still also producing higher linear alpha-olefins and polymeric materials.
WO 2009/006979 A2 describes a process and a corresponding catalyst system for the di-, tri- and/or tetramerization of ethylene, based on a chromium complex with a heteroatomic ligand, typically featuring a PNPNH-backbone and activated by an organoaluminum compound such as, e.g., trialkylaluminum or methylaluminoxane. Among other possible embodiments of this invention, CrCl3(thf)3 (thf=tetrahydrofuran) is preferentially used as chromium source.
EP 2 239 056 A1 describes a catalyst composition and a process for oligomerization, in particular for the selective trimerization of ethylene to 1-hexene, using a modification of the catalyst system disclosed in WO 2009/006979 A2. While also relying on ligand types featuring the PNPNH-backbone, these modified systems show distinct advantages over the original catalyst compositions in terms of stability, activity, selectivity and the allowable window of operability concerning process parameters in a technical environment.
According to EP 2 239 056 A1, halogen-containing modifiers are used, in conjunction with, for example, Cr(acac)3 (acac=acetylacetonate), the PNPNH-ligand and triethylaluminum as activator. Typical modifiers are, e.g., tetraphenylphosphonium- or tetraalkylammonium halides, preferentially the chlorides. In contrast to catalyst systems using CrCl3(thf)3 as chromium source, these modified systems allow for a free and independent adjustment of the chromium/halogen/aluminum ratio. This is a very advantageous strategy, since basic mechanistic investigations have shown that the halogen is an indispensable constituent of the catalytically active species, thus influencing the overall catalytic performance.
A typical oligomer product distribution of this above mentioned catalyst system is:
C42.9wt.-%C691.4wt.-% (>99.0 wt.-%) 1-hexeneC80.5wt.-%C105.1wt.-%≧C120.1wt.-%
A typical process for a homogeneous catalyzed ethylene oligomerization technology of the prior art is shown in FIG. 1.
The homogeneous catalyst system 1 is transferred together with the solvent 2 (e.g. toluene) to the reactor 3. The linear alpha olefins, mainly 1-hexene, are formed via trimerization of dissolved ethylene in the liquid phase. Within the reactor the reaction heat of the exothermic reaction has to be removed and a fast phase transfer of the gaseous ethylene to the solvent has to be realized. Various reactor types are conceivable. Some examples are:    1. Bubble column reactor: to avoid internal heat exchange surfaces, ethylene can be used both as reaction feed and cooling medium. Simultaneously, mixing is achieved via the rising bubbles above a suitable sparger plate.    2. Loop reactor with external heat exchanger.    3. Plug-flow reactor: the reaction heat can be removed via the reactor wall.
A preferred reactor for the ethylene oligomerization is the bubble column reactor. Ethylene is introduced via a gas distribution system to the bottom section, whereas the liquid heavy LAOs, together with the solvent and the catalyst, are withdrawn from the bottoms. The oligomerization reaction is highly exothermic. By removing this heat with the ethylene, heat exchanger surfaces within the reaction area, which would be subject to heavy fouling, are avoided. A part of the formed linear a-olefins, which are gaseous under reaction conditions, are condensed at the top of the reactor and serve as reflux for cooling purpose, taking advantage from their respective heat of evaporation. Typical reaction conditions are: 30-70° C. at 10-100 bar.
After the reaction section the liquid product including the solvent (e.g. toluene) with the dissolved ethylene is fed to the separation section. In a first column 4 the unconsumed ethylene is separated from the product and the solvent. The ethylene is recycled back to the reactor via line 5. Ethylene polishing 6 may take place at line 5. The heavier fractions are routed to the subsequent separation 7 where they are divided into the different fractions (C4, C6, solvent, C8, C10, >C12). The solvent is recovered and recycled back to the reactor.
Starting with the class of very advantageous modified catalyst systems, as described, for example, in EP 2 239 056 A1, the question arises how an economic process for the oligomerization of ethylene, especially the selective trimerization of ethylene to 1-hexene, should be designed. The following challenges have to be considered in this regard:    1. The reaction heat from the exothermic reaction has to be removed from the reactor. Due to the fact that the catalyst is very sensitive against high temperatures, a reaction temperature preferably between 30 and 70° C. has to be maintained and controlled very precisely.Through the fact that small amounts of polyethylene are formed during the reaction, internal heat exchange surfaces show a tendency towards fouling. This leads inevitably to an unstable and/or unsafe operation of the reactor at only very limited times-on-stream. Therefore, such internal heat exchange surfaces should be avoided.    2. Unfortunately, formation of polymer or high molecular weight oligomers cannot be avoided completely during ethylene oligomerization, since this is an inherent side-reaction channel.
These solid materials may either be dissolved or suspended in the liquid product and, thus, finally be passed to the separation section or they may deposit in the inner surface of the reactor and its peripheral equipment. The latter is the worst case, since this may lead to fouling and plugging of the reactor. Consequently, the reactor and its associated equipment have to be cleaned periodically to remove the deposits. This leads to shutdowns and consequently to production loss. Consequently, polymer which is dissolved or suspended in the product stream is preferred.